OPA992-Q1: If I do not power the OP-AMP, is it safe for current to flow through Vin- and Vo?

DeWei Lan

Prodigy 250 points

Part Number: OPA992-Q1

Here is my TIA circuit

Since this circuit has high feedback resistor (100Meg) for OP-AMP,

Vo would be about 300mV when power off due to environment light,

and Vo would be about 100 mV when power on due to 0.1 bias of V+,

as this picture:

Is it safe to have >200mV crossover between Rfb(Vo, Vin-) if power is off?

I recently had 9 channel failures on my circuit board.

I thought maybe that was the problem.

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 286800 points

The absolute maximum ratings would allow up to 10 mA through the clamping diodes.

What happens to PD_RevBias when power is off?

I would suspect ESD or other overvoltage. Are there any cables/connectors that can be touched or hot-plugged?

0 DeWei Lan over 1 year ago in reply to Clemens Ladisch

Prodigy 250 points

Hi Clemens Ladisch:

Thank you for your reply.

The photodiode's datasheet says its maximum current is 0.25mA.

Clemens Ladisch said:
What happens to PD_RevBias when power is off?

I use a PSU to power both RevBias and Vs+, so when the power goes out RevBias is cut off as well.

Clemens Ladisch said:
I would suspect ESD or other overvoltage. Are there any cables/connectors that can be touched or hot-plugged?

I will check our operator whether notice that. Thank your for your suggestion.

Best Regards!

0 Gerasimos Madalvanos over 1 year ago in reply to DeWei Lan

TI__Mastermind 29146 points

Hey DeWei,

I agree with Clemens comments, connectors are a common point of ESD/EOS.

Additionally, I would check the power sequencing of the PSU. If V+ is turned off before PD_RevBias, there is still potential for damage.

Best,
Jerry

0 DeWei Lan over 1 year ago in reply to Gerasimos Madalvanos

Prodigy 250 points

Hi Gerasimos Madalvanos:

Thank you for your reply.

Gerasimos Madalvanos said:
Additionally, I would check the power sequencing of the PSU. If V+ is turned off before PD_RevBias, there is still potential for damage.

I think up! I adopt positive LDO is TPS742 which configured with 10ms soft-start time.

But negative LDO for PD_RevBias has not soft-start(TPS72325).

Could it be because of the reason you mentioned?

Gerasimos Madalvanos said:
I agree with Clemens comments, connectors are a common point of ESD/EOS.

Actually, my circuit only electrical connection with power and test point to scope.

My power input has been installed ESD protection diode.

So I guess there's probably not much of a problem here.

Maybe the operator accidentally touched the circuit when power on?

BTW, next version we will remove PD_RevBias as it is not needed after validation.

Best Regards!

DeWei

0 Gerasimos Madalvanos over 1 year ago in reply to DeWei Lan

TI__Mastermind 29146 points

Hello DeWei,

If there is a current path from PD_RevBias to IN- of the device where more than 10mA can flow, this could damage the device.

Best,
Jerry

0 DeWei Lan over 1 year ago in reply to Gerasimos Madalvanos

Prodigy 250 points

Hi: Gerasimos Madalvanos:

As SFH-250V datasheet said, maximum photocurrent is 100uA

I confirm a thing:

If voltage or current at OP-AMP pins can't over absolute maximum ratings,

then can Vin+ or Vin- pass current or voltage even though the OP-AMP is unpowered?

Best regards!

De Wei

0 Gerasimos Madalvanos over 1 year ago in reply to DeWei Lan

TI__Mastermind 29146 points

De Wei,

The input protection structure of the op amp looks like the below diagram:

If a voltage is present on IN+ or IN- with no voltage on the supply, the diodes will be forward biased and conduct current. This will then try to find some path to ground, either through the op amp, the ESD Cell, or some other parasitic path. For more information on this, please see the TI Precision Labs video on ESD structures.

https://www.ti.com/video/4080362747001

Best,
Jerry

Amplifiers

Amplifiers forum

OPA992-Q1: If I do not power the OP-AMP, is it safe for current to flow through Vin- and Vo?